fixes #7222; fixes #5595; fixes #3747

* late instantiation for the generic procs' default param values
* automatic mixin behaviour in concepts

Other fixes:

* don't render the automatically inserted default params in calls
* better rendering of tyFromExpr

tests/generics/tlateboundgenericparams.nim

@@ -0,0 +1,124 @@
+import typetraits
+
+type
+  Foo = object
+
+proc defaultFoo: Foo = discard
+proc defaultInt: int = 1
+proc defaultTInt(T: type): int = 2
+proc defaultTFoo[T](x: typedesc[T]): Foo = discard
+proc defaultTOldSchool[T](x: typedesc[T]): T = discard
+proc defaultTModern(T: type): T = discard
+
+proc specializedDefault(T: type int): int = 10
+proc specializedDefault(T: type string): string = "default"
+
+converter intFromFoo(x: Foo): int = 3
+
+proc consumeInt(x: int) =
+  discard
+
+const activeTests = {1..100}
+
+when true:
+  template test(n, body) =
+    when n in activeTests:
+      block:
+        body
+
+  template reject(x) =
+    static: assert(not compiles(x))
+
+  test 1:
+    proc t[T](val: T = defaultInt()) =
+      consumeInt val
+
+    t[int]()
+    reject t[string]()
+
+  test 2:
+    proc t1[T](val: T = defaultFoo()) =
+      static:
+        assert type(val).name == "int"
+        assert T.name == "int"
+
+      consumeInt val
+
+    # here, the converter should kick in, but notice
+    # how `val` is still typed `int` inside the proc.
+    t1[int]()
+
+    proc t2[T](val: T = defaultFoo()) =
+      discard
+
+    reject t2[string]()
+
+  test 3:
+    proc tInt[T](val = defaultInt()): string =
+      return type(val).name
+
+    doAssert tInt[int]() == "int"
+    doAssert tInt[string]() == "int"
+
+    proc tInt2[T](val = defaultTInt(T)): string =
+      return type(val).name
+
+    doAssert tInt2[int]() == "int"
+    doAssert tInt2[string]() == "int"
+
+    proc tDefTModern[T](val = defaultTModern(T)): string =
+      return type(val).name
+
+    doAssert tDefTModern[int]() == "int"
+    doAssert tDefTModern[string]() == "string"
+    doAssert tDefTModern[Foo]() == "Foo"
+
+    proc tDefTOld[T](val = defaultTOldSchool(T)): string =
+      return type(val).name
+
+    doAssert tDefTOld[int]() == "int"
+    doAssert tDefTOld[string]() == "string"
+    doAssert tDefTOld[Foo]() == "Foo"
+
+  test 4:
+    proc t[T](val: T = defaultTFoo(T)): string =
+      return type(val).name
+
+    doAssert t[int]() == "int"
+    doAssert t[Foo]() == "Foo"
+    reject t[string]()
+
+  test 5:
+    proc t1[T](a: T = specializedDefault(T)): T =
+      return a
+
+    doAssert t1[int]() == 10
+    doAssert t1[string]() == "default"
+
+    proc t2[T](a: T, b = specializedDefault(T)): auto =
+      return $a & $b
+
+    doAssert t2(5) == "510"
+    doAssert t2("string ") == "string default"
+
+    proc t3[T](a: T, b = specializedDefault(type(a))): auto =
+      return $a & $b
+
+    doAssert t3(100) == "10010"
+    doAssert t3("another ") == "another default"
+
+  test 6:
+    # https://github.com/nim-lang/Nim/issues/5595
+    type
+      Point[T] = object
+        x, y: T
+
+    proc getOrigin[T](): Point[T] = Point[T](x: 0, y: 0)
+
+    proc rotate[T](point: Point[T], radians: float,
+                   origin = getOrigin[T]()): Point[T] =
+      discard
+
+    var p = getOrigin[float]()
+    var rotated = p.rotate(2.1)
+